Tar Spot: 2022 Update
By: Justin Hunter, Sales Agronomist
Tar spot in corn has been a hot topic in the Midwest for the last few years. Most farmers in eastern Iowa have probably heard of the detrimental yield impact it has caused in areas of Illinois but have yet to see it affect their crop – so most are probably thinking, ‘what’s the big deal?’ In order to acknowledge the threat of tar spot, one must first understand how the disease disperses, infects and damages the crop.
Tar spot prefers cool, humid conditions with extended periods of leaf wetness. Leaf wetness is the key for most crop diseases. The more rain we receive usually equates to more disease in the field. The same general rule can be made about tar spot. This rule can help explain why Iowa has yet to see a big yield hit since the inoculum first became widespread across the state in 2019. The inoculum lives in corn residue and is dispersed through wind. In 2019, tar spot was present in about 75% of Iowa (Robertson, 2019). Today, it is present in every county. Crop rotation is likely to help lessen severity but will not prevent infection from taking place, and tillage has not shown to be an effective control option.
Once conditions are right and infection takes place, tar spot lesions will start appearing in two weeks. They resemble black splatter marks of tar that are raised from the leaf tissue. The most distinguishing characteristic is that these spots cannot be rubbed or scraped off. Spots can occur as early as the V8 growth stage. The major concern comes when infection takes place early – that is when up to 50% yield loss becomes possible. Luckily for local farmers though, the disease has been appearing late enough to not cause much yield damage.
The disease produces a toxin that rapidly kills leaf tissue – leaving the plant with less leaf area for photosynthesis. As the disease progresses on the canopy, the ear starts pulling sugars from elsewhere (i.e., stalks and roots) during grain fill. This process leaves the plant more vulnerable to other diseases and affects stalk integrity. This past year, farmers in the Stutsman territory may have noticed standability issues in their corn because of tar spot.
Fortunately, there are control options available that could help limit the effects of this disease. The most reliable control option is to apply a two-mode fungicide at VT/R1. The premium fungicides with long-lasting residues are the best options, but any fungicides that are labeled for tar spot will help. There is not much known about hybrid tolerance at this point. Tar spot cannot be cultured, so the development of tolerant hybrids is difficult to study and produce.
During the 2022 growing season, it would be wise to make fungicide a priority. We do not know when tar spot will start robbing big yields in Iowa, but we do know that this disease will not be going away any time soon.
